End mills, such as a square end mill, a radius end mill, and a ball-nosed end mill, are known in which a gash is formed in the top of a flute formed in the periphery of a top portion of an end mill body. The gash has a shape which is like a shape formed by cutting out a wall face of the flute which faces an end mill rotating direction. The wall face of the flute facing the end mill rotating direction is an end cutting edge rake face, and an end cutting edge is formed along a top side ridge portion of the wall face.
For example Japanese Unexamined Patent Application Publication No. 2005-125433 (“the '433 Publication”) which is incorporated by reference herein in its entirety, discloses end mills in which an intersecting ridgeline is positioned between the end cutting edge rake face and the gash bottom face of the gash which faces the top side of the end mill body. Another intersecting ridgeline is positioned between this gash bottom face and the gash wall face of a gash which faces the rear side in the end mill rotating direction. A cross-section, which is perpendicular to a direction in which at least one of the intersecting ridgeline extends, shows that this intersecting ridgeline portion has a form like a concave curved shape. The concave curve formed in the cross-section of the intersecting ridgeline has a radius of curvature, and the radius of curvature at the outer peripheral side of the end mill body is longer than that at the inner peripheral side of the end mill body.
Also, Japanese Unexamined Patent Application Publication No. 2008-44038 (“the '038 Publication”), which is incorporated by reference herein in its entirety, discloses a ball-nosed end mill in which a gash having a V-shape in a cross-section perpendicular to the flute of the gash is provided at a hemispherical portion of an axial top, and a radius cutting edge is formed along this gash. A cutting-edge-rear-side side wall, which is one of several side walls paired with each other in the V-shaped gash, is opposite to the radius cutting edge. The side wall has a concave surface which is smoothly concave in a cross-section perpendicular to the flute of the gash.
In the end mill described in the '433 Publication, one intersecting ridgeline is positioned between the end cutting edge rake face and the gash bottom face, and also the other intersecting ridgeline is between the gash bottom face and the gash wall face. If one of the above two intersecting ridgelines has a form like a concave curved shape in a cross-section perpendicular to the direction in which the intersecting ridgeline extends, the other intersecting ridgeline must have a form in which the end cutting edge rake face and the gash bottom face intersect each other at a narrow angle, as if a face were sharply bent; and/or the gash bottom face and the gash wall face intersect each other at a narrow angle, as if a face were sharply bent. Therefore, chips which are generated by the end cutting edge and flow along the end cutting edge rake face may cause clogging that hinders smooth chip discharging.
Additionally, in the above case where the end cutting edge rake face and the gash bottom face intersect each other at an angle, and/or the gash bottom face and the gash wall face intersect each other at a narrow angle; stresses tend to concentrate at the intersecting ridgeline which in concentration may cause a crack. Thereby, the top portion of the end mill body formed between the gashes may be fractured. In addition, problems similar to the above problems also exist in the end mill described in the '038 Publication, since, in this end mill, the concave surface formed on the cutting-edge-rear-side side wall intersects the radius-cutting-edge-side side wall at a narrow angle or may be connected with the radius-cutting-edge-side side wall by a concave circular-arc having a small radius.
In the end mill described in the '433 Publication, the intersecting ridgeline between the end cutting edge rake face and the gash bottom face and the intersecting ridgeline between the gash bottom face and the gash wall face have a form similar to a concave curved shape in a cross-section. In particular, FIG. 3 of the '433 Publication shows that the gash bottom face itself becomes a concave circular-arc shape with a large radius of curvature, by which the end cutting edge rake face and the gash wall face are smoothly connected to each other. In such a case, chips can flow smoothly and the stress concentration does not arise.
In this case, however, since the end cutting edge rake face and the gash wall face need to be connected together by a concave circular-arc having a large radius of curvature, the width of the gash increases. Additionally, the thickness (back metal) of the top portion of the end mill body decreases, such that the top portion loses its good rigidity. The top portion of the end mill body is located on the rear side of the end cutting edge in the end mill rotating direction, and is formed between the gashes. This problem becomes especially significant in an end mill in which the number of end cutting edges is three or more, the number of gashes is three or more, and the top portion thereof is largely cut out to form the gashes.